With contactless power transmission, detecting a conductor such as metal or a circuit including a coil near power transmission/reception coils, if any, is very important for power charge with high safety and efficiency.
Using a combination of power transmission and reception devices, any metal substance inserted between the power transmission device (power transmission coil) and the power reception device (power reception coil) has been previously detected based on information about amplitude and phase at the time when the power reception device shows a change of load (for example, see PTL1). Any metal substance has also been detected based on a change of power transmission/reception efficiency (also referred to as coil-to-coil efficiency), or based on a change of sensor output using a magnetic sensor, a capacitive sensor, an infrared sensor, or others.